The present invention is directed to devices and methods for oxygenating blood during surgery, and in particular, to membrane oxygenators.
Devices which oxygenate blood are typically used in surgical operations where the supply of blood from the heart is interrupted, e.g. during open heart surgery, or any other surgery performed on the heart or lungs. Oxygenation devices fall generally into two different categories, bubble and membrane oxygenators.
For reason not to be discussed herein, membrane oxygenators have recently become preferred over bubble oxygenators. Membrane oxygenators include a membrane with which the blood is brought into direct contact. Commercially available membrane oxygenators utilize hollow tubing or fibers which are formed from a material through which gas may diffuse under the proper operating conditions, without an appreciable passage of fluid. The blood is either passed through the tubing or fibers, with an oxygen bearing gas passed about the fibers, or conversely the blood can be passed about the tubing or fibers with the oxygen bearing gas passed there through. The tubing or the hollow fibers may be formed from silicon, e.g. silicon tubing, or be formed as a porous fiber from a hydrophobic polymeric material.
Examples of various types of membrane oxygenators are disclosed in U.S. Pat. No. 4,261,951, issued to Milev on Apr. 14, 1981; U.S. Pat. No. 4,376,095, issued to Hasegawa on Mar. 8, 1983; U.S. Pat. No. 4,424,190, issued to Mather, III et al, on Jan. 3, 1984; and U.S. Pat. No. 4,657,743, issued to Kanno on Apr. 14. 1987, and European Patent Application Number 176,651, filed by Mitsubishi Rayon Co. Ltd, on Feb. 14, 1985.
The gas exchange rate for an oxygenator is determined by multiplying the mass transfer constant for that specific oxygenator times the partial pressure of oxygen, times the total gas exchange surface area. The mass transfer constant is dependent upon the material from which the membrane is formed, the porosity of the membrane, and the operating characteristics of the oxygenator, that is, the manner in which the blood is passed across the membrane. The mass transfer constant is fixed for each type of oxygenator. While the partial pressure of oxygen can be altered to affect gas transfer, it can not exceed one atmosphere and is usually fixed for each type of oxygenator. Thus, the only variable which can be altered to effect the overall gas exchange rate for a given oxygenator is the total gas exchange surface area.
It is thus evident that in order to increase the gas exchange capabilities for a specific oxygenator, its size must be increased. While this allows for a greater gas exchange, the resulting oxygenator may suffer other drawbacks. For example, the larger the oxygenator the greater the priming volume. The priming volume is that amount of fluid necessary to fill the oxygenator. Generally, it is desirable to minimize the priming volume which reduces the gas exchange surface area, while designing an oxygenator with a higher mass transfer constant.
To a certain degree the material from which the membranes are formed, as well as the dimension and porosity of such membranes, may be controlled to improve the mass transfer constant. Altering the operating characteristics of an oxygenator has the greatest effects upon the mass transfer constant. Generally, this involves altering the manner in which the blood is passed across the membranes.
Some blood oxygenators provide that the blood will flow in a direction substantially equivalent to the axis of the hollow fibers, see U.S. Pat. No. 4,698,207, issued to Bringham et al on Oct. 6, 1987. This is a result of directing the blood either through the fibers, or in a direction generally equivalent to the orientation of the hollow fibers. The basic disadvantage with this type of construction is the inefficiency in performing the oxygenation as blood flows across the membrane.
Other blood oxygenators attempt to direct the blood across the width of the fibers, see U.S. Pat. No. 4,424,190, issued to Mather, III et al on Jan. 3, 1984. The oxygenator disclosed in this reference incorporates an internal porous cylindrical core 68 about which is positioned a plurality of hollow fibers. The core is formed with numerous openings orientated in a direction generally perpendicular to the hollow fibers. The core and fibers are positioned within an outer cylindrical wall 66, with gas directed through the individual hollow fibers. Blood enters core 68, and exits out through the openings in a direction generally perpendicular to the bundle of hollow fibers. The direction of blood flow through the device taught in Mather III, et al provides for a better degree of oxygenation than obtained with those devices which pass the blood in substantially the same direction as the axis of the hollow fibers. However, the flow obtained by the device taught in Mather III, et al is uneven, and tends to become concentrated in certain portions of the fiber bundle.
An integrated blood oxygenator and heat exchange device is disclosed in a recently published Japanese patent application, publication number 1988-139562 which was published Jun. 11, 1988. The blood oxygenator includes a central heat exchange core which is formed with laterally disposed folds, that is folds arranged between the core ends. Numerous hollow threads are wrapped about the heat exchange core. These threads are laid either parallel to the core axis or piled one by one diagonally across the core. The blood is directed into the core folds and then passes out through the fibers. There is no commercially available embodiment of the disclosed device which would allow for a direct comparison of the gas exchange rate.
There thus remains the need to provide a membrane oxygenator having an improved efficiency in the exchange rate for oxygen.